As designers and manufacturers continue to shrink the size of circuit components, the shapes reproduced on the substrate though photolithography become smaller and are placed closer together. This reduction in feature size and spacing increases the difficulty of faithfully reproducing the image intended by the design layout onto the substrate and can create flaws in the manufactured device. To address the problem, one or more resolution enhancement techniques are often employed to improve the resolution of the image that the mask forms on the substrate during the photolithographic process.
One of resolution enhancement techniques, “optical proximity correction” or “optical process correction” (OPC), tries to compensate for light diffraction effects. When light illuminates the photomask, the transmitted light diffracts, with light from regions with higher special frequencies diffracting at higher angles. The resolution limits of the lens in a photolithographic system make the lens act effectively as a low-pass filter for the various spatial frequencies in the two-dimensional layout. This can lead to optical proximity effects such as a pull-back of line-ends from their desired position, corner rounding, and a bias between isolated and dense structures. The optical proximity correction adjusts the amplitude of the light transmitted through a lithographic mask by modifying the layout design data employed to create the photomask. For example, edges in the layout design may be adjusted to make certain portions of the geometric elements larger or smaller, in accordance with how much additional light exposure (or lack of exposure) is desired at certain points on the substrate. When these adjustments are appropriately calibrated, overall pattern fidelity is greatly improved, reducing optical proximity effects.
Numerical noise caused by computation in an optical proximity correction process can lead to different corrections for the same layout pattern in different locations. While the deviations are usually small, it is desirable for an optical proximity correction tool to produce a consistent result for the same layout patterns, which is particularly true for designs with repetitive patterns such as memory cells.